― A Little Sister, singing to the tune of "Frere Jacques" while harvesting.[src]

Little Sisters (originally named as Gatherers by the game developers) are young girls who have been genetically altered and mentally conditioned to reclaim ADAM from corpses around Rapture. Little Sisters are almost always accompanied by a Big Daddy. They generally are immune to damage, outside of being "harvested", and have no offensive abilities. Attacking them, however, will incur the wrath of their Big Daddy protectors.

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ADAM is excreted by unique Sea Slugs found near Rapture. The ADAM producing Sea Slugs did not naturally produce a large enough quantity for serious research and commercial exploitation. Dr. Brigid Tenenbaum developed a procedure whereby a Sea Slug was implanted in a human host's stomach, and a symbiotic interaction between host and slug yielded up to thirty times the quantity of usable ADAM. Female children, eventually known as Little Sisters after implantation, were found to be the only viable hosts. Although Tenenbaum hoped to be able to keep the children in a vegetative state, it was found they had to be fully functional to produce the ADAM. They still maintained many of the ordinary characteristics of small children: laughing, playing, smiling, and singing.

Frank Fontaine created his Little Sister's Orphanage in Apollo Square and other locations as a front to exploit the children as his supply of hosts for ADAM production. When Andrew Ryan took over Fontaine Futuristics, he discovered the existence of the children. He found them appalling, but had to accept the necessity of their existence to generate the quantity of ADAM Rapture now required. It wasn't long before the entire city became aware of where ADAM was really coming from. He shut down those false orphanages, and continued to use the Little Wonders Educational Facility to house the existing ADAM hosts. With the City's ADAM addiction endemic, and ADAM's use to fight the civil war, shortages were becoming critical. Eventually, as the civil war's body counts grew and the shortages increased, a solution to get additional ADAM led to using the children to obtain ADAM from corpses. Little Sister ADAM hosts were mentally and physically prepared for this grim harvesting role. Ryan, to negate the abhorence of the population, portrayed the Little Sisters in a marketing gimmick, their image being used on Plasmid and Tonic vending machines called Gatherer's Gardens, and even produced toy dolls of them. Rapture public announcements introduced them as the salvation of the City, a reassurance of continuing ADAM production.
With the growing chaos of the Civil War, additional little girls started to be kidnapped directly from their parents.

"Remember Kids!" is a series of three instructional videos aimed at Little Sisters seen inBioShock 2. The first two videos contain the hidden messages "Father" and "Mommy".

“Who watches over sleeping angels? I do, I do…”

― A Little Sister as she starts to harvest an ADAM corpse in BioShock 2.[src]

Dr. Yi Suchong, realizing that the need for ADAM was greater than the amount produced by the Little Sisters themselves, pioneered a way for them to recycle ADAM from the blood of dead Splicers, greatly increasing their ADAM yield.[1] Naturally, the children were unwilling to cooperate. To overcome this basic obstacle, Dr. Suchong created methods to mentally condition the children. This produced a neurological impulse to recognize the bodies of dead Splicers as "Angels" to make the grisly process of extraction and consuming their ADAM-rich blood appealing for the girls.[2] Like many of the tools in Rapture, the Little Sisters' ADAM extractor syringes were fabricated from common items: large needles, hose nozzles, and baby bottles.

To make Little Sisters more effective in their task, they were heavily conditioned to see the world of Rapture very different from reality. Instead of seeing the dark, deteriorated state of the city, they saw an idealized child's world, with rosy marble floors, pink or red drapings, and flowering vines growing everywhere. Little Sisters saw their Big Daddies as noble golden knights, and saw Big Sisters as brides or princesses with elegant ballroom apparel. Statues appeared to them as heroic Big Daddies, pools of blood as rose petals, flies as butterflies, random fires on the floor as lit fireplaces, weapons as red plastic toy guns, and the dead bodies of Splicers as "Angels"; men and women lying in elegant repose with the outline of a halo and angelic wings surrounding them. Splicers themselves appeared as handsome and beautiful, taking the forms of elegantly dressed men and women in masquerade masks, conversing civilly, when in reality they were violently fighting. This "dream world" persisted in the girls' day-to-day experience, except for occasional lapses where their senses were brought abruptly back to the grim, twisted reality of Rapture's decay whenever they were frightened or startled. This was also shown to happen while concentrating on the process of gathering ADAM from a corpse. However, this could just be due to Delta temporarily losing control over the girl while she harvests ADAM.

The Alpha Series were a failed line of prototype protectors developed first by Dr. Yi Suchong. The line was completed by Dr. Gilbert Alexander using prisoners from Persephone Penal Colony as test subjects. The main difference between the Alpha Series and the other Big Daddies is the nature of their bond to the Little Sisters. Each Alpha Series was created with an unbreakable physiological bond to a single Little Sister. This bond created a fail-safe enforced by each Little Sister's pheromone signature — an Alpha Series that wandered too far from his bond partner for an extended period of time would lapse into coma or madness.

This bond proved to be too effective, however, as each Big Daddy in the Alpha Series became useless after his Little Sister was killed by attacking Splicers or saved by Brigid Tenenbaum. Any Alpha Series model that was not rendered comatose as a result of his Sister being lost descended into emotional instability to the point of psychosis. When the Alpha Series failed, it was discovered that previous Big Daddy models such as the Bouncer and Rosie could be used as more suitable protectors.

With physical and neurological drives in place, the Little Sisters eagerly pursued gathering ADAM in Rapture. The ADAM coursing through their bodies made them virtually indestructible, immediately repairing any damaged tissue. However, they still had the stature and strength of small girls (not to mention their constant hallucinatory state); thus, they were a constant target for abduction and exploitation. Dr. Suchong, in light of the rising mortality rate of Little Sisters, initiated the Protector Program. The idea was to create a series of protectors that would defend the Little Sisters from Splicer attacks so they could gather ADAM without interference. This ambition proved to be more of a challenge than anticipated by the scientists, and numerous attempts were needed to find the right balance of ADAM-based conditioning in both the gatherer and the protector. Despite the tests and trials, through the hands of a third party, the final key to the bond was found to be quid pro quo, as both discover that one could not survive without the other. The results of the program had each Little Sister paired with a Big Daddy, a heavily spliced human test subject grafted into an enormous armored diving suit. After an unfortunate event leading to the demise of Dr. Suchong, the bond was further strengthened through conditioning employing pheromones. The pheromone produced by the Big Daddy attracted the Little Sisters, and its counterpart produced by the Little Sisters instinctively drove the Big Daddies to defend them with their lives. Once Big Daddies came into full production, the Little Sisters were also put through a course of Pavlovian psychological exercises,[3] designed to force them into rejecting all positive feelings towards any mother-type figure, and instead place all of their affection upon a generic Big Daddy archetype. As a result, the little girls see every Big Daddy protecting them as the same person, and consider the mere mention of the word 'Mommy' to be taboo.

Because of the pheromone and Suchong's conditioning regimen, the Little Sisters are strongly attached to the Big Daddies, affectionately calling them a number of pet names such as 'Mister Bubbles' or 'Mister B'. When a Big Daddy is killed, its Little Sister will stop whatever she is doing and mourn its death. However, the strong bond between the pairs made the Big Daddies highly aggressive, prompting Ryan to issue public warnings against approaching any Little Sisters to prevent any accidents involving citizens or interference.

At one point, McClendon Robotics was tasked with creating a line of Robotic Little Sisters to supplement the human Little Sisters. However, testing trials revealed that the Big Daddies simply ignored the robotic gatherers, leaving them vulnerable to attacks by Splicers.[4] This forced the team to abandon the project, and left an entire production line of Robotic Little Sisters discarded in the offices of McClendon Robotics.

Removal of the Sea Slug from the host under normal circumstances is fatal. Although the host doesn't die immediately (their heartbeat can still be heard), it was likened to taking a patient off life support: their demise was inevitable. In a forced removal, a large amount of ADAM could still be obtained from the Sea Slug. However, Tenenbaum's constant contact with the Little Sisters and their unsuppressed childlike behavior eventually caused her to seek a way to safely remove the sea slug. In time, she was able to design a Plasmid with the ability to restore some characteristics of the Little Sister's pre-Sea Slug state. This is, however, only a physical change. For example, the appearance and voice are returned to a natural state, they lose their ADAM-producing ability and strengthened healing factor, and are less prone to enragement and fear, recognizing the player as a friend. However, they still retain results of their mental conditioning, still treating Big Daddies as parental figures, seeing the world in a dream-like wonderland with angels, and instinctively using Vents or using their extractors to drain corpses of ADAM. Presumably, the mental conditioning requires a different cure. Tenenbaum established a safe house for restored Little Sisters in the tunnels beneath Olympus Heights for that purpose.

After the events of BioShock, Tenenbaum left for the surface, taking many of the saved Little Sisters with her. However, Sofia Lamb soon came into power in Rapture and later restarted the Little Sister program to supply the Rapture Family with ADAM, and to further her own plans. There were not enough young girls left in Rapture to supply the ADAM cycle, and many of those who remained had matured to the point that they were only useful as Big Sisters.

Lamb began a program of kidnapping young girls from the surface to convert them into Little Sisters. Taken by Big Sister protectors, these new Little Sisters were processed in the Paediatric Wing of the Persephone prison colony under the watchful eye of Edward Grimes and his staff of physicians. The new girls developed a unique psychological bond with Eleanor Lamb, who could see through their eyes and even control them to a certain extent.

When Tenenbaum discovered the kidnappings on the surface, she knew immediately where the girls were being taken, and what purpose they were to serve. She acquired a submersible and descended to Rapture expecting to find the missing girls turned into Little Sisters, gathering alongside Big Daddies. So she began rescuing them again. Eventually, Eleanor contacted Tenenbaum through her connection to the Little Sisters, and they created a plan to save the Little Sisters and help Eleanor escape to the surface by resurrecting Subject Delta. After Delta was brought back to life using a Vita-Chamber in the Adonis Luxury Resort, the Little Sisters began to work with him, helping him to reach Eleanor at Persephone. Meanwhile, Tenenbaum reached out to other deactivated Alpha Series Big Daddies throughout Rapture, such as Subject Sigma, and asked for their aid in rescuing the remaining Little Sisters in the city.

Each Little Sister is a young girl between five and ten years of age.[10] In BioShock, the Little Sisters had varying hair colors, but in BioShock 2 they are all brunette, including Cindy Meltzer in game due to time constraints preventing the developers from making her blonde. The girls are clothed in dirty dresses of various colors that reach just below their knees. All of the Little Sisters are barefoot while they explore the halls of Rapture, but before the riots they wore shoes, as evidenced in the opening cutscene of BioShock 2 and Burial at Sea. Due to their fast regeneration, they can walk on any harsh surface without problems.

Evidence of the Little Sisters' healing factor.

The Little Sisters are nearly invulnerable to damage while they have the Sea Slugs within them. When one is struck her flesh will glow an unnatural golden color, showing the effects of fast regeneration due to ADAM. However, even with the fast regeneration of ADAM the girls are still vulnerable. The Little Sisters could be killed by Splicers who wanted to extract the ADAM they contained by forced removal of the Sea Slug (i.e. harvesting).[11] Although Little Sisters are "invincible" and able to regenerate any damage, this invincibility is not perfect. An example of this is if the Little Sister were to break her legs, forcing them at odd angles, they would heal so quickly the bones would fuse at these angles. To be set right, they would have to be broken repeatedly due to the almost instant healing process. Furthermore, Little Sisters are not immune to the resulting pain.[12]

The only attack that can affect Little Sisters is the splash damage of a grenade. When a Little Sister is in the vicinity of a thrown grenade, she will become ragdollish and move with the splash impulse as she is briefly knocked unconscious, then stand back up. Little Sisters can even survive being underwater. In BioShock 2, there is one instance where the player can see Little Sisters underwater. If the player rescued all of the Little Sisters in BioShock 2, then they are used by Eleanor to help boil away the water in the ballast chamber of Augustus Sinclair's lifeboat. In that scene, the Sisters stay underwater for a prolonged period as they boil the water.

2

3

4

5

There are a total of 21 Little Sisters which can be rescued or harvested, disregarding any extra Little Sisters which may appear as a result of glitches. If the player harvests all 21 of these Little Sisters, they will gain a total of 3360 ADAM throughout the game. If the player rescues all of the Little Sisters, and finds all of the gifts from Dr. Tenenbaum, they will earn a total of 3080 ADAM and a number of Gene Tonics. Accessing the start menu will reveal the number of Little Sisters in the player's current level, as well as their current status.

Only the second Little Sister which can be rescued or harvested is protected by a Big Daddy.

A Little Sister can briefly be seen from the windows of the long tunnel joining the Medical Pavilion to the Surgery Foyer. This Little Sister keeps walking around a tunnel with her Big Daddy. This Little Sister cannot be reached and therefore cannot be interacted with.

The Little Sister which the player needs to protect is not counted because she cannot be rescued or harvested, since she is no longer implanted with a slug. If the Splicers kill this Little Sister, a new Little Sister can be summoned from a vent an indefinite number of times.

After fighting Fontaine, the player can see Little Sisters coming out of their vents and stabbing Fontaine to death with their needles.

There are also 7 Little Sisters that might bring the teddy bear gift rewards during the game, but none of these Little Sisters can be rescued or harvested.

The numbers in brackets are for Little Sisters that cannot be interacted with in normal gameplay.

In total, there are 21 Little Sisters whom the player can rescue or harvest or leave alone throughout the course of the game. If one exploits the Extra Little Sister Bug at every occasion, this total extends to 25.

Every three Little Sisters rescued nets the player a reward in the form of a teddy bear clutching a present left at a nearby Gatherer's Garden. The cured Little Sister who delivered the present can sometimes be seen running to the nearest vent.

Harvesting represents the short-term solution: Not only will the player end up with more ADAM than if they had saved the Little Sisters, but the first few levels of the game will be much easier to achieve. Coupled with the potential for sampling many different Plasmids and Tonics at once, this is the ideal strategy for someone playing BioShock for the first time. However, the drawback is that this will lead to the unhappiest ending to the game.

Saving, on the other hand, is a long-term investment. While it is harder to survive the first few levels with this path, it ultimately offers greater rewards. The Tonics and Plasmids the player receives are among the most powerful in the game and cannot be obtained otherwise. The ADAM gap between a Harvester and a Rescuer (280 ADAM in total) is also compensated by these bonuses. However, these rewards may or may not suit one's play-style, so it is recommended to either look up the rewards individually or play the game as a Harvester first. This option is recommended for experienced players or for those who have already played the game. This strategy will also unlock the happiest ending in the game.

Mixing both strategies is not excluded. By both harvesting and saving the Little Sisters the player offers themselves a generous dose of ADAM, with unique Plasmids and Tonics to boot. The player only gets their rescue reward after saving three Little Sisters, so if one is feeling scrupulous do so in blocks of three so as to get that reward as quickly as possible. This strategy will never get the player as much ADAM as a Harvester, nor as many unique rewards as a Rescuer. However, with efficient planning and harvest/rescue alternation one can combine the advantages of both paths, making one's life easier during the first stages of the game while being able to use a Big Daddy as a bodyguard. This option can be taken by any type of gamer, regardless of their skill or experience level. This path will also offer the player an alternate ending to the game, similar to that of the Harvester ending but less miserable.

In BioShock 2, the tactics of gaining ADAM are similar to the first game. Once the player comes across a Little Sister, Subject Delta can either Harvest her or Adopt her. Little Sister rarely carries much ADAM when she is first encountered. However, the player can choose to pick her up and look for an ADAM-rich corpse to gather more ADAM from. This will attract the Splicers' attention, forcing the player to set up a perimeter to defend the Little Sister while she gathers. Once she has finished harvesting the ADAM, the Little Sister will give the player the ADAM she has gained. After that, the player has a choice of either saving the girl and sending her through a Little Sister Vent back to Tenenbaum, or harvesting her, since the girl will contain a lot more ADAM after gathering.

This system is meant to be a harder choice for the player, since the potential gain from harvesting is much greater. However, by saving Little Sisters, the player can earn a present, including a Tonic called Proud Parent, increasing the ADAM harvested per corpse by 20, and the Demanding Father Tonic that increases the speed at which the Sister gathers ADAM.

Additionally, in BioShock 2 the Little Sisters have an added ability. They are able to heal their wounded Protectors over time. When a Big Daddy loses health from attacks by Splicers or Subject Delta, the Protector's bonded Little Sisters can refill his health by injecting ADAM from their syringes into his body through the palm.

To acquire a Little Sister, her Big Daddy protector must be killed first. Afterward she stalls for a few seconds before recognizing and greeting Delta (if he is a rescuer), or reacting with fear to his presence (if he is a harvester). This is due to Eleanor Lamb's psychological affinity for Subject Delta. Because of her past as a Little Sister and her additional ADAM augmentation, Eleanor Lamb possesses a psychological link to all the Little Sisters. This allows her affection for Subject Delta to transfer over to every Little Sister and allows the player to adopt any Little Sister in Rapture. As a result, the Little Sisters call Subject Delta "daddy," and treat him as if he were their true father instead of calling him "Mr. Bubbles" or "Mr. B" as they do the other Big Daddies.

Near the end of the game, Delta is captured by Sofia Lamb and is immobilized. To help him escape, Eleanor Lamb sends a Little Sister to him with a Plasmid to take control of her. Upon the Little Sister injecting Delta with the needle, the player will take on the viewpoint of the Little Sister. During this, the whole level will look pristinely beautiful, an illusion, since the Little Sister's mental conditioning affects her perception of the world around her. The nightmarish prison cells and environment will be completely replaced by spotless white curtains, plush red carpeting, and toys. Splicers will be replaced by masked men and women in tuxedos and ballroom dresses, blood stains by flower petals, etc.

Depending on the choices the player has made throughout the game, the Little Sister may see different statues throughout the level depicting Delta's actions. The only times the view will show the level's reality is when moving toward a Splicer held in a cell, harvesting from corpses, picking up parts of a Big Sister suit, and heading back to Sofia Lamb's office. Once Eleanor has been given the suit parts, she will save or harvest the Little Sister Delta was controlling (depending on the player's actions towards the Little Sisters), returning him to his body.

The player's choice of whether to save or harvest Little Sisters affects the ending of the game and how Eleanor treats the other Little Sisters.

2 Little Sisters are seen in a flashback when Subject Delta is captured by Sofia Lamb inside the quarantine chamber where her daughter Eleanor is held. One is seen reviving Delta at the Vita-Chamber in Adonis Luxury Resort at Eleanor's request in the game's beginning, and the other is the Sister previously rescued/harvested at Ryan Amusements.

A Little Sister comes out of a vent and is then controlled by the player, she can collect ADAM and other objects for Subject Delta. This Little Sister is then rescued or harvested by Eleanor.

A Little Sister throws a ball from a vent, as Subject Delta enters the Therapy Wing.

6 Little Sisters are rescued or harvested by Eleanor Lamb in the Pediatric Wards. If rescued, these 6 Little Sisters enter the water in the ballast tank under the escape vessel, to help boil the water.

10 Little Sisters can be seen with Eleanor Lamb in the final 'Good Ending' cut scene.

There are also 3 Little Sisters that bring the teddy bear gift rewards during the game, but they cannot be rescued or harvested.

The numbers in brackets are for Little Sisters that cannot be interacted with in normal gameplay.

There are a total of 12 Little Sisters with whom the player can interact in the game.

A reward is hidden in a teddy bear at a nearby Gatherer's Garden for every four Little Sisters rescued. A bug sometimes prevents the Little Sister from moving away until Delta is right next to her. In BioShock 2, there are three gifts in total, each granted after four Little Sisters have been saved. In Minerva's Den, there are only two, although they are each rewarded after saving three Little Sisters.

While the harvest/save dilemma is still present in the sequel, a new mechanic is introduced: adoption. This section will cover the latter strategy. For information on the two other mechanics, see the previous section.

Each Little Sister can harvest up to two corpses each. Since ADAM is the most precious resource in the game and can only be found in (very) limited supply, it is heavily recommended to have the Little Sister harvest as many corpses as possible.

Each successful harvest yields 40 ADAM, meaning the player can harvest up to 80 ADAM per Sister by adoption alone. Use of the Proud Parent Tonic increases the gain from 40 to 60 ADAM per corpse and use of the Master Protector tonic increases the gain from 40 to 80 ADAM per corpse (only in Minerva's Den). The final amount given upon harvesting or rescuing the Little Sister is unaffected.

When having spotted a corpse, secure the entire area: hack every machine, equip the right Tonics (especially Proud Parent, Master Protector and Demanding Father, if the player has them), fill up on ammo, health and EVE, and buy enough First Aid Kits and EVE Hypos to be able to wage a small war. Next up, lay traps (Trap Rivets, Trap Spears, Proximity Mines, etc.) on those passages and around the corpse. If one has any Mini-Turrets, deploy them too. If the player considers themselves sufficiently prepared, signal to the Little Sister that she can start harvesting.

Upon adopting a Little Sister, an icon to the middle left of the HUD will indicate the Little Sister's status: if the Little Sister is on Delta's shoulders, the icon will show a Little Sister standing in front of a Big Daddy. When the Little Sister starts harvesting, a progress bar will appear, showing how much time the player has left before the Little Sister will have finished extracting ADAM from the corpse (thus ending the waves of Splicers). When harvesting, the icon will change to a Little Sister sticking her needle into a corpse. If the Little Sister is being assaulted by a Splicer the icon will change to a vivid exclamation mark, and the progress bar will pause until the Little Sister is freed from her aggressor.

The best form of weaponry when dealing with multiple waves of Splicers, besides traps, is ordnance and Plasmids capable of dealing very quickly with several enemies, for example the Machine Gun or the Launcher. However, it is recommended to conserve resources and use the cheapest ammunition types if possible.

After every fourth Little Sister is rescued or harvested a Big Sister will come to kill Subject Delta. It is best to be fully prepared for this, as the Big Sister is a difficult enemy to defeat. An alley, passage, corner of a room, doorway or similar structure is a good site to set up a defensive position.

In BioShock 2, a Rescuer can gain up to 3240 ADAM (assuming the player follows the full adoption routine for each Sister and collects every Sea Slug) while a Harvester will earn a total of 3330 ADAM, creating a gap of 90 ADAM. In the long run, this means a Harvester will be able to buy more and better Plasmids, although a Rescuer will be better stocked up on resources and will have the Demanding Father Tonic to boot. However, the difference isn't as marked as in the first game, and both alternatives here are equivalent.

If one chooses to harvest the Little Sisters after obtaining the Proud Parent Tonic, one will earn a total of 3410 ADAM, 80 more than what a pure Harvester would receive, coupled with a few extra resources.

In Minerva's Den, the dilemma is slightly more straightforward, as harvesting after getting the Proud Parent Tonic will still get the player 20 less ADAM than if one were to harvest all of the Little Sisters. Although the gap between a Rescuer and a Harvester is much larger (140 ADAM), a Rescuer will also receive the Armored Shell 2 Gene Tonic, one of the most useful in the game and unattainable otherwise. As the final fight between Sigma and Reed Wahl will require the player to soak up a lot of physical damage, this may prove invaluable. However, a Harvester will be able to outfit themselves with slightly better Plasmids and Gene Tonics, thereby compensating for the loss somewhat.

The player has the option of earning all the stars in the Protector Trials before playing Minerva's Den, not only for the 480 ADAM boost the Master Protector Tonic offers, but also as it provides the player with training on how to deal with waves of enemies during a gathering session.

In the Multiplayer, Little Sisters feature in several game modes as a capture-able objective. In Capture the Sister, ADAM Grab, and Team ADAM Grab, combatants compete to defend or abscond with a Little Sister in a war-torn district of Rapture. In the matches, capturing a Little Sister and depositing her into a vent earns the player the Little Sister Capture Reward yielding 100 ADAM.

Little Sisters can be seen being trained outside a Little Wonders facility in Market Street. Judging from their non-glowing eyes, the girls have not yet been implanted with ADAM slugs. They do appear to have been put through some mental conditioning, however, as they stare at their surroundings in a dream-like trance.

Prior to the civil war, Little Sisters may have worn pancaked makeup to hide their identities from their previous parents/guardians, who may attempt to take them back by force.

When Elizabeth is later forced to search Dr. Suchong's Free Clinic during the early days of the civil war, she finds two Little Sisters, Masha and Leta, in the facility's secret research laboratory. She convinces the two little girls to save a dying Big Daddy by using their ADAM to heal him, and by doing so successfully created the 'Pulling the Thorn from the Lion's Paw' bonding method.

In the PC version of BioShock 2, if the player chooses to bind a different key for "use" they will still be prompted to press the original "use" key when adopting a Little Sister, but this will do nothing. Pressing the key that has been re-bound to "use" will allow the player to adopt a Little Sister (this is fixed in the 1.50 PC Patch for BioShock 2).

In BioShock 2 Multiplayer, when one plays Capture the Sister, ADAM Grab or Team ADAM Grab, The player often picks up the Sister and if one is killed, then they drop her. The Little Sister can be dropped on places that will cause her to be in the "falling state" permanently, and there is no way to pick her up when she's falling. Ex: if one would drop her on the lamp over one of the surgery beds in Dandy Dental of Medical Pavilion, she will get in that state. This will be undone when she re-spawns after 1 minute.[13]

Early in the development of BioShock, the player was to collect ADAM directly from the sea slugs that produce it; the slugs even had their own Gatherer vent on the ground. However, the developers wanted to give the choice of harvesting ADAM more impact, so the host design went through many changes, including a small chipmunk like creature (like in the Utropolis story), but the effect was deemed too comedic. The concept of Little Sisters stemmed from the need to find a more empathetic victim for the player.[14]

Originally the developers had planned that the Little Sister could be killed by physical damage,[15][16][17][18] but in the games they are nearly invulnerable, despite a burnt model existing in the game files of BioShock.[19] This is explained by the regenerative ability of the Sea Slugs within them. However, the little girls will still be immune to damage even after they have been rescued by the player character. Attempting to harm them as they run to the nearest vent will have no effect. Although they can be knocked down by explosions from enemies after being rescued, they inevitably get back up and continue to the closest vent.

In BioShock the Little Sisters are voiced by Juliet Landau.[20] In BioShock 2 the Little Sisters are voiced by Jodelle Ferland[21] (who is much closer to the apparent age of the Little Sisters).

Concept art of a Little Brother.

Brigid Tenenbaum once wondered why they were only able to use little girls instead of boys. She was given a sarcastic answer by Fontaine that it meant one less bathroom to build in the orphanage.[22] Despite this, in the Deco Devolution Artbook for BioShock 2, it is mentioned that "Little Brothers" were a concept explored by the developers, but were not represented in the games themselves. They would have been housed in the Therapy Rooms of Inner Persephone.

In BioShock: Rapture the novel, when Tenenbaum and Suchong first present a Little Sister to Fontaine, they mention also creating a Little Brother, but reveal that the experiment did not end well.

The developers originally allowed the Multiplayer characters the option to harvest or adopt the Little Sister. This was likely to gain an extra ADAM reward, and would only have functioned in a gamemode which was removed.[23]

In a Reddit AMA, Ken Levine mentions how the Little Sister pair bonding relationship was inspired by that of ants: "The original idea came to me when I was watching a show about ants. ( I think it was ants). There were the worker/gatherer ants, the army ants who protected them and other bugs who ate ants. I remember thinking that those roles read very clearly even if you didn't know anything about the story and how cool it would be to be in a game where you could watch that ecology around you. It took some time for the ants to anthropomorphize into Big Daddy and Little Sister."[24]

In demo, the Little Sister would cough up blood after gathering but this was cut from the final version of the game.

The Little Sisters were partly inspired by the Grady Twins from the 1980's classic film: The Shining.[25]

Lead Animator Shawn Robertson did motion capture by himself for the first Little Sister Jack can choose to save or harvest in BioShock.[26] His daughter also did motion capture for Sally's appearance in Paris in Burial at Sea - Episode 2.[27]

Concept art for the Robotic Little Sister.

The Robotic Little Sisters featured in Minerva's Den were actually made as a possible substitute to the regular Little Sisters for the German version of BioShock (since murdering children in a video game was unacceptable in Germany) but was left unused.[28]

Jeff Weir, 2K Marin Animation Supervisor, explains why the Little Sisters changed appearance from BioShock to BioShock 2: "This game focuses more on the father/daughter relationship. The first game had a much creepier Little Sister. This time we want you to feel more like she could actually be a real girl. There are places in the Eleanor story about how horrible her life was and this feeling that they were real children and that they were taken against their will for the most part as opposed to being these creepy gremlin kids that were always there and always kind of disturbing."[29]